1. Field of the Invention
The present invention relates generally to portable chain saw sharpening devices, and is particularly concerned with devices of this type in which the sharpener body is provided in the form of an inverted U-shaped member adapted to be placed over the bar of a chain saw.
2. Description of the Prior Art
Power-driven chain saws are widely used both for commercial purposes, as for example in clearing areas of land of trees and brush in preparation for the construction of buildings and dwellings, and for non-commercial purposes such as cutting firewood for home use or pruning unwanted branches from trees. A problem that has always confronted chain saw users, however, is that of maintaining the necessary degree of sharpness in the cutting edges of the saw chain. Without periodic sharpening, the cutting links of the saw chain eventually become dull with use, requiring additional pressure on the chain saw to effect the desired cut. A particular problem is presented when the teeth on one side of the saw chain are dull in comparison to the teeth on the other side, as often happens when the saw chain is brought into contact with a material other than wood (e.g., a nail or a piece of fence wire) during cutting. This condition will cause the chain saw to deviate from a straight cut, resulting in crooked cutting. A similar result occurs when the teeth on one side of the saw chain are longer than those on the other side of the saw chain.
In the past, large specially-designed machines have been provided for sharpening the saw chain used on chain saws. However, inasmuch as these machines usually require removal of the saw chain from the remainder of the chain saw, they are rather inconvenient to use. Then, too, machines of this type are not always available to non-commercial or occasional users of chain saws, such as farmers and homeowners. For this reason, it has been common among such users to hand-sharpen the cutting edges of the saw chain using an ordinary round file. This procedure is rather tedious and time-consuming, particularly since the orientation of the file must continually be changed to accommodate the oppositely-facing cutting links that alternate on the saw chain. Moreover, the hand-sharpening procedure does not always produce uniform sharpening of all the cutting links on the saw chain. Another drawback of hand sharpening is that the file can etch longitudinal scratches on the cutting surfaces, which then tend to retain sap and dirt from the wood being cut. As the scratches fill, the cutting surfaces become dull and additional downward pressure is needed to cause cutting. This, in turn, gives rise to frictional heat build-up which can cause annealing or softening of the saw chain teeth.
In an attempt to remedy these problems, the so-called "self-sharpening" chain saw has been developed. Chain saws of this type typically include a self-contained stone grinding wheel which is brought into contact with the saw chain as the latter travels around the saw bar. The inherent problem with this approach is that the grinding wheel is altered each time that it is used due to the gradual wearing away of the cutting abrasives. As a result, the configuration of the cutting teeth changes as the grinding wheel ages, and consistent and uniform sharpening of the saw chain becomes impossible with time.
Perhaps the most useful kind of chain saw sharpener that has been proposed, at least from the standpoint of the average non-commercial user not having access to sophisticated sharpening machinery, is the portable chain saw sharpener of the type which can be attached directly to the saw bar for sharpening the cutting teeth when required, and then removed. Since the abradant sharpening element is not part of the chain saw itself, it can be replaced as necessary to assure a consistent degree of sharpening. Moreover, since sharpeners of this type usually include some sort of aligning or positioning fixture for the abradant sharpening element, the possibility of unequal sharpening as between the different cutting teeth on the saw chain is largely avoided. Most importantly, sharpeners of this type are usually compact and inexpensive devices which are economically attractive to noncommercial users of chain saws.
A large variety of portable chain saw sharpeners, sometimes taking the form of simple filing guides or templates, have been proposed in the prior art. See, for example, the following United States patents:
______________________________________ 2,594,821 3,172,306 2,677,289 3,172,307 2,755,559 3,283,615 2,762,241 3,438,286 2,770,985 3,543,612 2,813,438 3,670,600 2,871,728 3,744,349 3,060,768 3,889,556 3,071,987 ______________________________________
A particularly useful type of portable chain saw sharpener is disclosed in U.S. Pat. No. 2,770,985, to A. W. Pearce. The sharpener consists of a channel-shaped frame with spaced parallel walls and clamping screws for affixing the sharpener to the bar of a chain saw in a straddling manner. The walls are provided with two pairs of aligned bores arranged diagonally with respect to the frame. A rotatable abrading implement is removably engaged in one or the other of the two pairs of aligned bores to sharpen the saw teeth positioned between the two walls. The walls are connected by a web at the top of the sharpener which is provided with a sight opening through which the saw teeth are visible during sharpening. By engaging the rotary abrading implement first in one of the two pairs of aligned bores and then in the other, the two sets of oppositely-directed cutting teeth on the saw chain can be sharpened without removing the sharpener from the saw bar.
Notwithstanding the utility and simplicity of the foregoing arrangement, at least two problems still remain. In the first place, in the absence of any means for positively locating the cutting link within the body of the sharpener prior to and during sharpening, consistent and uniform sharpening of all the cutting links is not assured. Moreover, the disclosed sharpener includes no provision for filing the depth gauge portions of the cutting links, which are essential in maintaining the proper depth to which the cutting edges penetrate the wood being cut. If the depth gauges are set too high, the cutting edges will be unable to penetrate sufficiently, requiring increased downward pressure on the chain saw to enable cutting. If they are set too low, the penetration of the cutting edges into the wood will be too great, causing grabbing and jerking of the chain saw.
More recently, in U.S. Pat. No. 3,744,349, to A. A. Juncker, a portable chain saw sharpener has been proposed in which the guide bores are provided in a movable carriage mounted on one side of the sharpener frame, rather than in the form of pairs of aligned bores penetrating both side walls of the sharpener as in U.S. Pat. No. 2,770,985. A system of guide bars and screws is provided in the device of U.S. Pat. No. 3,744,349 to allow adjustment of the carriage in both the horizontal and vertical directions. In addition to the two clamping screws used for affixing the sharpener body to the bar of the chain saw, a third clamping screw is provided for securing a chain tooth temporarily relative to the sharpener frame. Once the tooth is clamped in position, the end of a tapered carbide steel file is inserted into one of the guide bores and the carriage is adjusted until the file touches the edge of the tooth. The carriage is then locked in position and sharpening is carried out by advancing the file, which now is rotated by a power drill, across the front of the tooth and back. The next tooth is then advanced to the same position, which is established by an adjustable stop means in the form of a reversely-bent leaf spring protruding into the sharpening area. The sharpening operation is then repeated. When all of the teeth facing in one direction have been sharpened, the file is inserted in the second guide bore and the process is repeated for the teeth facing in the other direction. In order to allow for grinding of the depth gauges, the carriage is provided with a third guide bore disposed above the first two. After sharpening, a further adjustment of the carriage is made to bring the third guide bore into alignment with the depth gauge of the first tooth, whereupon the turning file is inserted into the third guide bore to grind the depth gauge to the desired level. This operation is then repeated for the depth gauges of the remaining teeth.
Although the sharpener described by U.S. Pat. No. 3,744,349 possesses the undeniable advantage of combining the sharpening and depth gauge filing functions in a single device, there are several disadvantages inherent in its design and its intended manner of operation. In the first place, the need for a movable carriage to adjust the position of the guide bores adds an undesireable degree of complexity to the device, as compared to the essentially unitary sharpeners of the type described, for example, by the previously-noted U.S. Pat. No. 2,770,985. Further, the described procedure of using the carbide steel file for adjusting the position of the guide bores relative to the cutting teeth, relying as it does on bringing the tooth into registry with some point on the gradually tapering end of the file, is inherently imprecise. The amount of sharpening will clearly depend upon how far the tapered file is inserted into the guide bore during the adjustment procedure, which is determined simply by visual approximation. If the approximation is incorrect, either too much or too little material will be removed from the cutting edge of the tooth during sharpening, and the adjustment must be repeated. Although the movable carriage assembly would no doubt facilitate such repeated adjustments, the adjustment procedure is still rather tedious and time consuming, since it amounts essentially to a trial-and-error process. Finally, and perhaps most importantly, the use of the file for adjusting the relative positions of the guide bores and the cutting teeth creates the risk of damage to the brittle carbide material of which the file is made. Such damage might occur, for example, as a result of inadvertently striking the file against the edge of a cutting tooth that is positioned too close to the guide bore before the adjustment is made. To the extent that such damage might necessitate premature replacement of the file, the user of the sharpener will be put to an additional and undesirable expense.